Remove unnecessary 'use' forms

Fix a laundry list of warnings involving unused imports that glutted
up compilation output.  There are more, but there seems to be some
false positives (where 'remedy' appears to break the build), but this
particular set of fixes seems safe.

src/libstd/iterator.rs

@@ -20,7 +20,6 @@ implementing the `Iterator` trait.
 use cmp;
 use iter;
 use num::{Zero, One};
-use num;
 use prelude::*;
 
 /// An interface for dealing with "external iterators". These types of iterators